Plastic-covered, ribbed, pictures that appear to move or change scenes are familiar to most Americans. These special effects can also include a three-dimensional effect. Lenticular sheets are laminated on to flat printed image. The cylindrical lenses are arranged in parallel strips and allow only one of the interlaced images to be seen by a viewer at any one angle. But as the angle of view changes, the scene can change and give the illusion of three-dimensions, morphing, animation, and flipping.
The lenticular sheets typically include a series of optical grade cylindrical lenses, or lenticules. In use, several images can be printed underneath the lenticules. Each image is divided into strips and one strip from each image is printed underneath each lenticule. The lenticules refract the different strips so that one of the images is visible at a time according to the angle of observation. Changing the angle of observation causes different images to appear and in consequence the image appears to change.
For lenticular printing to be effective the image strips must be correctly aligned with the lenticules. Sheets of lenticular film are typically manufactured for a specified number of lines (or lens) per inch rating which can range 6–300 lines per inch (lpi). The pitch of the lenticules is the distance between lenticule centers. Generally, the pitch of the lenticule is a function of the desired viewing distance. A higher pitch is used for longer viewing distances.
Effective lenticular printing requires extremely accurate pitch calibration of the lenticules, e.g., up to 1/100th of the lines-per-inch pitch. Due to imperfections in the manufacturing process, the actual pitch varies from the nominal value. Furthermore, it is important that the errors in the pitch of the interlaced print below the lenticules do not aggregate.